Evaluation of Increasing Dietary Threonine to Lysine Ratio in Corn Soybean Meal Diets with and without DDGS on Growth Performance and Carcass Characteristics of Growing-Finishing Pigs

A total of 2,160 pigs (PIC 337 × 1050; initial BW 77.3 lb) were used in a 112-d growth trial to evaluate the effects of normal or high SID Thr:Lys ratio in diets with and without DDGS on growth performance. Pigs were randomly assigned to pens (27 pigs per pen) in a randomized complete block design by BW with 20 replications per treatment. Pens of pigs were allotted to 1 of 4 dietary treatments arranged in a 2 × 2 factorial with main effects of dietary Thr level (Normal vs. High) and DDGS (with or without DDGS). Treatment diets were formulated in 4 phases from 75 to 125, 125 to 175, 175 to 230, and 230 to 300 lb BW. Diets with high DDGS were formulated to include 40% DDGS in phase 1 and 2, 30% in phase 3, and 15% in phase 4. Normal Thr diets were formulated to contain 61, 62, 63, and 65% SID Thr:Lys ratios for the four dietary phases, respectively. High Thr diets had SID Thr:Lys ratios of 67, 68, 69, and 72%, respectively. There were no interactions (P > 0.10) observed in any phase or overall, between Thr level and inclusion of DDGS for ADG, ADFI, F/G, and BW. For the overall period (d 0 to 112), pigs fed diets without DDGS had increased (P < 0.001) ADG and BW and reduced (P < 0.001) ADFI leading to improved (P < 0.001) F/G. There was no evidence for difference (P > 0.10) between diets with or without the inclusion of DDGS when diets were formulated to normal or high SID Thr:Lys ratio. In summary, the addition of high levels of DDGS reduced ADG and increased ADFI, which resulted in poorer F/G and lower final BW, regardless of the dietary SID Thr:Lys level. Additional research should be conducted to evaluate the effect of high Thr:Lys levels when soluble fiber sources are included in finishing pig diets instead of an insoluble fiber source such as corn DDGS.


Summary
A total of 2,160 pigs (PIC 337 × 1050; initial BW 77.3 lb) were used in a 112-d growth trial to evaluate the effects of normal or high SID Thr:Lys ratio in diets with and without DDGS on growth performance. Pigs were randomly assigned to pens (27 pigs per pen) in a randomized complete block design by BW with 20 replications per treatment. Pens of pigs were allotted to 1 of 4 dietary treatments arranged in a 2 × 2 factorial with main effects of dietary Thr level (Normal vs. High) and DDGS (with or without DDGS). Treatment diets were formulated in 4 phases from 75 to 125, 125 to 175, 175 to 230, and 230 to 300 lb BW. Diets with high DDGS were formulated to include 40% DDGS in phase 1 and 2, 30% in phase 3, and 15% in phase 4. Normal Thr diets were formulated to contain 61, 62, 63, and 65% SID Thr:Lys ratios for the four dietary phases, respectively. High Thr diets had SID Thr:Lys ratios of 67, 68, 69, and 72%, respectively. There were no interactions (P > 0.10) observed in any phase or overall, between Thr level and inclusion of DDGS for ADG, ADFI, F/G, and BW. For the overall period (d 0 to 112), pigs fed diets without DDGS had increased (P < 0.001) ADG and BW and reduced (P < 0.001) ADFI leading to improved (P < 0.001) F/G. There was no evidence for difference (P > 0.10) between diets with or without the inclusion of DDGS when diets were formulated to normal or high SID Thr:Lys ratio. In summary, the addition of high levels of DDGS reduced ADG and increased ADFI, which resulted in poorer F/G and lower final BW, regardless of the dietary SID Thr:Lys level. Additional research should be conducted to evaluate the effect of high Thr:Lys levels when soluble fiber sources are included in finishing pig diets instead of an insoluble fiber source such as corn DDGS.

Introduction
Threonine is an essential amino acid for pigs. Mucus in the gastrointestinal tract has a high concentration of Thr. Dietary fiber increases endogenous protein losses by increasing sloughing of intestinal mucosal cells, increasing mucus production, and making the Thr requirement for maintenance relatively high. 3 Consequently, pigs fed high fiber diets could require an increase in the SID Thr:Lys ratio in the diet. Therefore, the objective of this study was to determine if an increased Thr:Lys ratio is required to optimize growth performance when DDGS is included in the diet of grow-finish pigs.

Materials and Methods
The Kansas State University Institutional Animal Care and Use Committee approved the protocol used in this experiment. This experiment was conducted in two barns at a commercial research grow-finishing site in southwest Minnesota (New Horizon Farms, Pipestone, MN). Each barn was naturally ventilated and double-curtain-sided with a slatted concrete floor and deep manure storage. Each pen (10 × 18 ft) was equipped with a 5-hole stainless steel dry self-feeder (Thorp Equipment, Thorp, WI) and a cup waterer for ad libitum access to feed and water. The facility was equipped with a computerized feeding system (FeedPro; Feedlogic Corp., Willmar, MN) that delivered and recorded daily feed additions.

Animals and diets
Two groups of 1,080 pigs (2,160 total pigs; PIC 337 × 1050; initial BW 77.3 ± 1.04 lb) were used in a 112-d growth trial. Pens of pigs (27 pigs per pen) were randomly assigned to 1 of 4 dietary treatments in a randomized complete block design with BW serving as the blocking factor resulting in 20 observations (pens) per treatment. Dietary treatments were arranged in a 2 × 2 factorial with main effects of SID Thr:Lys ratio (Normal vs. High) and DDGS (with or without DDGS). Diets were fed in 4 phases from 75 to 125, 125 to 175, 175 to 230, and 230 to 300 lb body weight (Table 1). Diets with DDGS were formulated to include 40% DDGS in phase 1 and 2, 30% in phase 3, and 15% in phase 4. Normal Thr diets were formulated to contain SID Thr:Lys ratios of 61, 62, 63, and 65%, and the high Thr diets had SID Thr:Lys ratios of 67, 68, 69 and 72%, for the four dietary phases, respectively. All treatment diets were manufactured at New Horizon Farms Feed Mill in Pipestone, MN, and were formulated to meet or exceed NRC 4 requirement estimates for growing-finishing pigs for their respective weight ranges (Table 1 and 2).
Pens of pigs were weighed, and feed disappearance was determined every 2 weeks to determine ADG, ADFI, and F/G. Two weeks before the end of the experiment, 3 pigs per pen were removed and marketed. The remaining pigs in the pen were weighed and marketed at the completion of the experiment. No carcass data were collected because of packing plant COVID-19 restrictions.

Statistical analysis
Pens of pigs were the experimental units for all data. Data were analyzed using the nlme package of R (Version 4.0.0, R Foundation for Statistical Computing, Vienna, Austria) as a randomized complete block design with body weight as the blocking factor and pen as the experimental unit. The main effects and interactions of DDGS and Thr level were tested. Model assumptions were checked and considered to be appropriately met. Differences between treatments were considered significant at P ≤ 0.05 and marginally significant at 0.05 < P ≤ 0.10.

Results and Discussion
There were no interactions observed between SID Thr:Lys ratio and DDGS for any response criteria (Table 3). Furthermore, no differences were detected (P > 0.10) for ADG, ADFI, F/G or BW for pigs fed diets with normal Thr levels compared to pigs fed high Thr throughout the study.
From d 0 to 28, pigs fed diets without DDGS had increased (P < 0.001) ADG, d 28 BW and improved (P < 0.001) F/G, with no evidence of difference (P > 0.10) in ADFI compared to pigs fed DDGS (Table 4). From d 28 to 56, pigs fed without DDGS had increased (P < 0.001) ADG and BW, and decreased (P < 0.012) ADFI that resulted in improved (P < 0.001) F/G compared to those pigs fed high DDGS.
From d 56 to 84, no evidence of difference was observed for ADG (P > 0.10) between pigs fed diets with or without DDGS, but pigs fed diets without DDGS had reduced (P < 0.001) ADFI and improved (P < 0.001) F/G compared with pigs fed DDGS. Interestingly, from d 84 to 112, pigs fed DDGS in the diet had increased (P < 0.001) ADG compared with pigs fed diets without DDGS, which could be explained by the reduction in the level of DDGS from 40 to 30 and 15% in the third and fourth dietary phase, respectively. However, pigs fed diets without DDGS had decreased (P < 0.001) ADFI with no evidence of difference (P > 0.10) in F/G between DDGS treatments. Overall (d 0 to 112), pigs fed diets without DDGS had increased (P < 0.001) ADG and BW, reduced (P < 0.001) ADFI, and improved (P < 0.001) F/G compared with pigs that were fed DDGS.
In summary, pigs fed diets with DDGS had decreased ADG, higher ADFI, and poorer F/G, resulting in 7.1 lb lower final body weight compared to pigs without DDGS in the diet. Increasing SID Thr:Lys ratio above pigs' requirement in this experiment did not improve final BW or overall growth performance. The results of this study suggest that feeding diets with high levels of DDGS decrease the overall growth performance with no ameliorating effect of increasing Thr in the diet. A potential reason could be that DDGS is an insoluble fiber source, different results might occur if a soluble fiber source is provided.
Brand names appearing in this publication are for product identification purposes only. No endorsement is intended, nor is criticism implied of similar products not mentioned. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.    A total of 2,160 pigs were used in two groups with 27 pigs per pen and 20 replicates per treatment. Phase 1 was fed from day 0 to 28, phase 2 from day 28 to 56, phase 3 from day 56 to 84, and phase 4 from day 84 to 112.